Since the hidden sector matter has feeble couplings with the Standard Model particles, they are
never in thermal equilibrium with the visible sector and the usual freeze-out analysis for the
computation of the relic density does not apply. However, the hidden sector particles can be
produced via the annihilation of the Standard Model particles into dark photons and
dark fermions via these feeble interactions and the computation of the relic density in this
case is computed using the freeze-in mechanism [1]}.
Within the dark sector itself the
dark fermions and the dark photons interact via normal size interactions
and are in thermal equilibrium up to a certain freeze-out temperature \(T_f\)  via
the process \(D\bar{D}\rightarrow \gamma ^{\prime }\gamma ^{\prime }\) . However, below the dark sector freeze-out temperature
\(T_f\) , the dark fermions and the dark photons decouple and the
dark photons decay to the visible sector much before the big bang nucleosynthesis, which leaves the dark fermions as the only DM candidates. In our model we assume that the visible and hidden sectors have the same temperature. One can consider sectors with different temperatures in the early universe, but it has been shown that the two sectors will eventually thermalize and that the effect on the relic density is minimal [2]}.
